1. Field of the Invention
The invention relates to a holding and positioning apparatus for an optical element, a manipulator unit for moving an optical element and a projection objective for semiconductor lithography.
2. Description of the Related Art
A projection objective, as disclosed by U.S. 2002/0167734 A1, for example, contains a beam splitter and a deflection mirror. While a light beam coming from a reticle is deflected by the beam splitter into a radial arm having a further mirror, the light beam coming from the radial arm is let straight through the beam splitter and deflected by the deflection mirror, parallel to the light beam coming from the reticle, into a further objective region.
In order that the light beam from the radial arm can be deflected into the objective region, it must be possible for the mirror plane of the deflection mirror to be displaced accurately into the point of intersection of the optical axes of radial arm and objective region. Furthermore, the normal to the mirror plane must be oriented toward the bisector of the angle which is formed by the optical axis of the radial arm and the optical axis of the objective region. For the positioning of the deflection mirror at the point of intersection of the optical axes of radial arm and objective region, it must be possible for the deflection mirror to be displaced in a translational manner normally with respect to its mirror plane. In order to orient the mirror plane normal to the bisector, it must be possible for the deflection mirror to be tilted about two mutually independent axes in the mirror plane.
By means of a subsequent change in the position or tilt angle of the deflection mirror, an image offset is primarily produced, the image being displaced with respect to the original image as a result. If vibration of the deflection mirror occurs, then the structures to be imaged in a projection objective for semiconductor lithography during the exposure process become blurred. This leads to an undesired loss of contrast and therefore to a reduction in the resolution of the objective.
Projection objectives for semiconductor lithography having holding and positioning devices for optical elements are known from the general prior art.
U.S. Pat. No. 5,986,827 discloses an actuator or a lifting and tilting apparatus for a workpiece. The actuator has three identical spring units, a structure on which the spring units are mounted, and three linear actuators. For the lifting and tilting mechanism, an inner ring, which carries the workpiece and has three bipods, as they are known, is provided, the latter each representing two rods arranged in a V shape and having spring joints at their ends, which are connected to one another at one end and which can transmit forces only along their longitudinal axis. The end of each bipod at which the rods are led together is in each case fastened to a lever, which is mounted in the structure or in the housing such that it can rotate and can be fixed with respect to the housing by setting screws. By means of tilting the respective levers with the setting screws, the inner ring together with the workpiece can both be adjusted vertically and tilted.
The disadvantage with the actuator and the lifting and tilting apparatus of U.S. Pat. No. 5,986,827 is the very large amount of space which is required, which results from the separation of bipod and actuating lever. Since, for production reasons, the bipods and the actuating levers have to be assembled from different parts, the bipod spring joints also have to compensate for the additional production and mounting tolerances, for which reason the bipods are relatively soft, so that the workpiece mounted by them can quite easily be excited to undesired oscillations.
Furthermore, U.S. Pat. No. 5,896,193 discloses a tilting and lifting apparatus having spring elements and at least one pneumatic cylinder. The pneumatic cylinder is arranged between an upper and lower structure and connected to the latter, in order to provide a constant rotational counterforce to the upper structure, which carries its own weight. The upper structure can be tilted and moved in the vertical direction and also relative to the lower structure. The upper structure is carried by the lower structure via three spring elements. Each spring element can move in the vertical direction but is kinematically stiff in the horizontal plane. Each spring element has three arms which are connected to one another in a Z shape. A drive unit is connected both to the spring element and to the lower structure. The upper structure is therefore very stiff in the horizontal direction. By means of an arrangement of this type, the spring elements are used to guide the upper structure in relation to the lower structure.
The disadvantage with such an apparatus is that, as a result of guiding the upper structure by means of the spring elements, the upper structure can be deformed and, as a result of the rectilinear guidance, the angles cannot be compensated.